Cathode-ray tubes (CRT) have been used for many years for the display of information such as texts and images in the electronics industry. Two important elements to be considered when designing CRTs are the construction of the electron gun and the phosphor screen which glows when struck by an electron beam.
A basic structure of a cathode-ray tube is shown in FIG. 1. The elements that make up the section of the electron gun is shown in more detail in FIG. 2. As shown in FIG. 1, cathode-ray tube 10 is constructed of an electron gun 12, a deflection coil 14 and a screen 8 coated with phosphor particles 16 contained in a vacuum glass tube 18. A cathode 20 (shown in FIG. 2) is heated and emits a stream of electrons 22 toward the screen 8. The cathode 20 in the CRT 10, in contrast to ordinary tubes which have cathodes that emit electrons from their sides, is designed such that it emits electrons primarily from the end facing the phosphor particle coated screen 8.
As shown in FIG. 2, the cathode 20 is enclosed in a metal cylinder which acts as a control grid 24. A small opening 26 at the center of the grid 24 which faces the phosphor coated screen 8 allows the electrons 22 to pass through. The small opening 26 forces the electrons to travel in a narrow beam due to its small size. The control grid 24 is normally charged with a negative voltage with respect to the cathode 20 so that some of the electrons emitted by the cathode are not allowed to pass through the opening 26. The intensity of the electron beam 22 is therefore controlled by changing the voltage to the control grid 24 with respect to the cathode voltage and determined by the difference between the two voltages. Since the more intense of the electron beam or the larger the number of electrons striking the phosphor, the brighter the phosphor screen will glow, the brightness of the display screen can be controlled by modulating the cathode-control grid voltages.
Once the electron beam 22 passes through the small opening 26 in the control grid 24, it moves through a second grid element 28, also called a focussing grid or focussing anode. The function of the focussing grid, which has a small opening 30 to allow the electron beam to pass through, is to further tighten the stream of electrons into a finer beam.
In a conventional CRT, as that shown in FIGS. 1 and 2, an electron gun 12 is constructed of several major components, i.e., a cathode 20, a cathode support 32, grids 24, 28 and a grid spacer 34. In the manufacturing process for an electron gun, it is difficult to produce a high quality, reliable focussing gun. This is especially true in a miniature CRT where the exact alignment of the two grids is very important. A typical manufacturing process can be carried out by first metalizing the grid spacer 34, and then welding two grids 24 and 28 to the spacer. Several drawbacks are inherent in this process. First, the process is complicated and requires the use of a large number of components. Secondly, the alignment of the two grids is difficult to carry out, and thirdly, since the position of the electron gun is not fixed in the CRT, any vibration of the tube will cause the formation of distorted images.
In miniature CRTs, these problems become more severe due to the small screen size, i.e., at between 0.5 inches and 1.5 inch, and the high resolution required to display texts and images on the screen. A miniature CRT must be capable of displaying information from VGA, SVGA, as well as video texts and TV images. They are normally used in applications where the CRTs are mounted on helmets, headbands and spectacles.
It is therefore an object of the present invention to provide an electron gun for a miniature CRT and a method of making the gun.
It is another object of the present invention to provide a one-piece grid assembly for use in an electron gun for a miniature CRT and a method of making the grid assembly.
It is a further object of the present invention to provide a one-piece grid assembly for an electron gun for use in a miniature CRT that has a simplified construction and a method of making the grid assembly.
It is yet another object of the present invention to provide a one-piece grid assembly for an electron gun for use in a miniature CRT that utilizes reduced number of components and a method of making the grid assembly.
It is still another object of the present invention to provide a one-piece grid assembly for an electron gun for use in a miniature CRT that does not require alignment between two grids and a method of making the grid assembly.
It is another further object of the present invention to provide an electron gun for use in a miniature CRT that can be securely mounted to the CRT such that any vibration of the tube would not cause image distortion and a method of making the electron gun.
It is still another further object of the present invention to provide a one-piece grid assembly for an electron gun for use in a miniature CRT wherein a control grid and a focusing grid are formed on a ceramic core and a method of making the grid assembly.
It is yet another further object of the present invention to provide a one-piece grid assembly for an electron gun for use in a miniature CRT wherein the grid assembly is formed by plating metal grids on a ceramic core.